In wavelength division multiplexed (WDM) systems, multiple optical channels, each at a different wavelength, are multiplexed and simultaneously transmitted through a single optical fiber. These optical signals need to be amplified (e.g., every 80 to 120 km) to compensate for optical power loss in the optical fiber.
Optical amplifiers utilizing rare earth-doped fiber amplifiers (e.g., EDFAs) in conjunction with optical pump provide optical power gain required to amplify all wavelengths simultaneously, thus lowering the cost of per channel amplification. Problems arise, however, when optical amplifiers do not provide uniform optical power gain to all of the transmitted wavelength channels.
Optical power variations between wavelengths, or spectral distortion, arises directly from optical amplifier gain (e.g., a non-uniform gain profile), and is further distorted by accumulated distortion. Furthermore, shorter wavelengths act as additional power pumps causing longer wavelength to experience additional gain known as Stimulated Raman Scattering (SRS). As a result, spectral tilt occurs with positive slope that continues to increase along the amplifiers chain.
In order to accurately correct and/or control spectral tilt in timely fashion, the tilt needs to be monitored and its slope accurately measured in real time. Conventional monitoring approaches use spectrum analyzers, wherein the spectrum is scanned or demultiplexed into individual wavelength channels where the optical power of each of the demultiplexed wavelength channels is measured separately. While spectrum analyzers are precise, they are also expensive and relatively large devices. Furthermore, while these monitors are accurate for measuring tilt caused by steady-state signal power variations, they are generally too slow for monitoring tilt caused by fast provision, optical channel restoration and optical power transients caused by fiber cut or equipment failures. All these events can cause significant spectral distortions and positive or negative tilt in a time scale of less than microsecond, rendering prior monitoring techniques ineffective in mitigating the negative impact of services.
Monitors for measuring spectral tilt caused by these fast transients are typically single-point monitors that only measure the total power of the optical signal. More specifically, these monitors estimate the spectral tilt using the linear relationship between SRS-induced spectral tilt and total power of the optical signal. These fast monitors, however, lack the spectral resolution and the determination whether the tilt has a positive or negative slope, information necessary to take corrective active.
It is an object of this invention to provide a spectrally resolved fast monitor for measuring spectral tilt.
It is a further object of the instant invention to provide a spectrally resolved fast monitor for measuring spectral tilt that is relatively compact and low cost.
It is a further object of the instant invention to provide fast feedback with accurate data to drive a tilt correcting device and/or drive the optical pump to adjust its optical pump power to a higher or lower magnitude as required.